Interislander expects to have passengers sailing across Cook Strait on its replacement ferry Stena Alegra by the weekend.

The company's general manager, Thomas Davis, said arrangements were in hand for the first freight-only sailing to leave Wellington at 5pm tomorrow night.

This would depend on the ship's crew successfully completing a further round of berthing trials in Picton today and Wellington tomorrow

If Interislander is happy with the return freight-only sailing on tomorrow night, it was possible up to 350 passengers could be boarding Stena Alegra in their cars for the 5pm sailing from Wellington to Picton on Thursday night, Mr Davis said.

The ship had met all Maritime NZ safety requirements during stringent inspections at Wellington's Aotea Quay.

The British-registered Stena Alegra will replace Aratere on the Cook Strait run for at least six months.

Aratere lost its starboard propeller on November 5 last year on a routine night sailing from Picton to Wellington when the starboard shaft snapped just outside Tory Channel at the top of the Marlborough Sounds.

Mr Davis said dry docking arrangements to fit two new shafts on Aratere had not been finalised. Contenders for the dry docking work included docks in Singapore, Malaysia, the Philippines and China.

"The tender for the repairs is yet to be let. We will be looking to award a [dry docking] contract later this month."

Mr Davis said a decision had been made to replace both the port and starboard propeller shafts as a precaution.